New Yorkers are seeing extra subway bag checks, vehicle checkpoints and more police officers throughout the city as U.S. intelligence officials have picked up threat information about possible al-Qaida terror attacks to coincide with the 10th anniversary of 9/11.

The NYPD has increased already-tight security because of the intelligence, which indicates a possible car bomb plot, officials said.

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What are intelligence officials doing with the information? (Published Saturday, Sep 10, 2011)

A joint FBI-Department of Homeland Security bulletin said al-Qaida may be considering attacks that use improvised explosives packed in vehicles, similar to the "attempted attack on Times square" by Faisal Shahzad in May 2010.

Al-Qaida may be aiming to avenge the death of Osama bin Laden and other key terror figures, the bulletin said.

He said he planned to go to the U.S. Open on Saturday, even as officials are concerned that sporting events and stadiums may be targets for car bombs.

Sept. 11 Anniversary in New York

Bloomberg, with Police Commissioner Ray Kelly and Janice K. Fedarcyk, the director of the FBI in New York, held a late-night briefing Thursday to warn New Yorkers to be vigilant and to expect even tighter security for the anniversary weekend.

The threat, Bloomberg said, "is credible, but it has not been corroborated. But we live in a world where we must take these threats seriously, and we will."

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Intelligence collected from overseas indicates a possible threat involving car bombs, as well threats to bridges and tunnels, according to a security official. The information indicated that three men would travel from Pakistan to the U.S. to carry out an attack.

The intelligence official said they are preparing for anything, but there is no weapons of mass destruction information on this threat. There are three elements to the threat, the source said, indicating some specificity on timing, location, and how it will be carried out, which explains the extra security.

New York City was "ramped up anyway for 9/11," the source said, but the threat has now brought a few hundred extra state police and National Guards troops to the streets.

The extra security sends a message of reassurance and deterrence, the source said.

Secretary of State Hillary Clinton said Friday that the threat "should not surprise any of us."

"It is a continuing reminder of the stakes in our struggle against violent extremism, no matter who propagates it, no matter where it comes from, no matter who its targets might be," she said, speaking in New York. "We are taking this threat seriously -- federal, state and local authorities are taking all steps to address it."

Investigators have been searching databases and travel records to see if anyone of concern entered the country in recent weeks that could match up with the threat information from overseas. Nothing has shown that matches the unconfirmed threat information.

One official said: "without names, we're stuck."

The information said one or two of the men might be traveling with U.S. passports, which complicates the search.

Officials are also visiting dealers who sell the kinds of chemicals that could be used to build a car bomb.

New York City and Washington, D.C., are mentioned in the non-specific car bomb threats, according to another source.

There will be increased towing of illegally parked cars, more bomb sweeps of parking garages, and extra police coverage on city ferries, he said.

The threat information has specificity and the source it comes from has been credible before, law enforcement sources said. But the sources stressed they do not know whether these three men exist or if they actually traveled to the U.S.

At Thursday's press conference, Fedarcyk said she would not get into the details of the source of the threat and whether it was "operational or aspirational." But she called it "specific" and "credible."

President Barack Obama and former President George W. Bush are set to attend the official 9/11 ceremony at ground zero on Sunday, and security was already expected to be tight downtown. Hundreds of surveillance cameras will be trained on the site, which will become a frozen zone on Saturday.

The mayor said the services making the anniversary would not be disrupted by the threat information.

"I can tell you our ceremonies will go on over the weekend exactly the way they were planned," Bloomberg said.

The public was urged to be extra-vigilant over the next few days and to report suspicious activity to authorities.

Whether or not the specific terrorists exist, an intelligence source told NBC New York that agencies are trying to answer that question, noting that there was enough information that merits such a heightened security response.

"It's frustrating this intelligence business sometimes," the source added.